Economical operation of high severity hydrocarbon cracking processes and equipment requires overcoming numerous competing operational and engineering challenges. The high temperatures and process stresses can exceed the long term viability of most conventional materials, including ceramics. In addition to component physical and thermal performance considerations, component chemical inertness and crystalline stability also become significant impediments requiring consideration. Component degradation and corrosion during long-term use present still further obstacles requiring address, particularly with regard to severe hydrocarbon processing.
One problem in the art pertains to ceramic stabilizer volatility and progressive loss of such stabilizer from the ceramic due to the severe pyrolysis temperatures and cyclic temperature swings. This stabilizer loss results in progressive reduction in crystalline stability and component degradation, eventually leading to premature component failure.
Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also known to be useful for cracking hydrocarbons and/or executing thermal processes, including some processes that are performed at temperatures higher than can suitably be performed in conventional steam crackers. As compared to conventional cracking equipment and processes, higher temperature reactions and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Properties such as temperature, reaction environment, component strength, and toughness limitations commonly defining upper limits for many of the processes and facilities.
In addition to processes utilizing high temperatures (e.g., >1500° C.), processes involving high temperatures plus large cyclic temperature swings and process fluid directional changes, such as regenerative or reverse flow reactor processes, pose even greater challenges. For example, the art discloses that to efficiently obtain relatively high yields of acetylene from pyrolyzing methane feed, such as in excess of 75 wt. % yield, reactor temperatures in excess of 1500° C. are required and preferably in excess of 1600° C., with relatively short contact times (generally <0.1 seconds). Due to the high temperatures involved, such processes are generally limited to relatively small amounts or batches using cyclical processes yielding a mixture of acetylene, CO, and H2. Due to the high severity, such methane cracking processes, however, have been relatively inefficient, impractical, and of very limited commercial value as compared to other more economical processes for generation of acetylene. Acetylene is typically generated commercially by cracking feeds is other than methane, which may be done at lower temperatures.
The high temperature processes (e.g., >1500° C.) have previously not scaled well and are generally only useful for relatively high-cost, specialty applications. Processes such as thermally cracking methane to acetylene have largely been commercially unattractive due in large part to thermal, chemical, and mechanical degradation of the reactor equipment, including ceramic materials used therein. Cyclic temperature changes and product flow direction changes impose severe physical strength and toughness demands upon the refractory materials at high temperature. Such stresses and performance demands have also typically limited manufacturing and use of the refractory materials to relatively simple shapes and components, such as bricks, tiles, spheres, and similar simple monoliths. Reactor component functions and shapes have been limited for high severity services.
In addition to physical temperature limitations for reactor materials, many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, leading to premature equipment degradation and/or process interference, such as by generation of unacceptable levels of contaminants in the process. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
The identified prior art pertaining to refractory materials for high-severity hydrocarbon pyrolysis dates primarily to the 1960's and earlier. However, that art merely occasionally provides generalized lists of some exemplary materials such as ceramics, alumina, silicon carbide, and zircon as reactor materials. These sparse, non-specific disclosures left the art largely incapable of providing a large-scale, commercially useful reactor or reactor process. The teachings of the art was only effective for enabling relatively small scale specialty applications that see vastly inferior use as compared to large scale processes such as hydrocarbon steam cracking. The identified art is void of teaching or providing a refractory ceramic material that is capable of sustaining the complex set of properties that are required for extended use in the reactive or other most-demanding regions of a high-severity (≧1500° C.) pyrolysis reactor, such as for the commercial production of acetylene and/or olefins from methane or other hydrocarbon feed. The studied art does not teach preferred crystalline structures or compositions for particular reactor furnace uses, or for complex reactor component shapes and/or functions. Multimodal ceramics are also is known in the ceramics art, as are ceramic compositions utilizing nanoparticles. However, the art remains void of teaching a ceramic or other composition or method of preparing the same that meets the rigorous performance properties needed for commercial application and long-term stability in high temperature cyclic pyrolysis processes. Further, the desired materials must maintain their formulations, crystalline structure, and corresponding physical and chemical properties for prolonged periods of time, at commercial scale and within the confines of an economic requirement. The studied art is believed to be similarly deficient at teaching materials suitable for complex, irregular, relatively fragile, or functionally-shaped reactor components.
For further example, the “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, such materials have demonstrated insufficient strength, toughness, and/or chemical inertness, and are not amenable to use as certain desirable reactor components, such as for use as reactor fluid conduits, to facilitate large-scale commercialization. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes (e.g., >1500° C., >1600° C., and even >1700° C.) has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors. Due to high temperatures involved in cyclic pyrolysis reactors, generally only ceramic components have the potential to meet the materials characteristics needed in such aggressive applications.
One attempt to overcome the above listed problems involved use of a “deferred combustion” process that delayed combustion and heat generation until the reaction components were positioned into the core of the reactor, thermally isolated from flow control equipment that was subject to premature degradation. The deferred combustion, regenerative reactor process and equipment was disclosed in a U.S. patent application filed Dec. 21, 2006, Ser. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems. Although the disclosed process of the '541 application effectively controls the location of combustion within the reactor, the internal reactor components must still contend with the severely high temperatures, temperature changes, and physical stresses incurred during methane pyrolysis, is particularly for a commercially desirable reactor life term. The refractory material comprising the reactive regions may typically be a ceramic or related refractory material. In some embodiments, however, the disclosed processes and apparatus may utilize relatively complex shaped refractory components, such as a thin-walled honeycomb monolith used to conduct process fluids through the reactor. Such reactors and reactor component geometries may demand materials that have strength, toughness, chemical inertness, and other required properties that exceed the capabilities of previously identified or known refractory materials under such temperature and stress conditions.
Ceramics components generally can be categorized in three material categories: engineering grade, insulation grade, and refractory grade. The term “engineering grade” has been applied to ceramic materials which typically have very low porosity, high density, relatively high thermal conductivity, and comprise a complete component or a lining. Examples include dense forms of aluminum oxide (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), silicon aluminum oxynitride (SIALON), zirconium oxide (ZrO2), transformation-toughened zirconia (TTZ), transformation-toughened alumina (TTA), and aluminum nitride (AlN). These materials usually possess high strength and toughness, which have been dramatically improved to the degree that ceramics are now available that can compete with metals in applications previously thought impossible for ceramics. Strength is a measurement of the resistance to formation of a crack or structural damage in the material when a load is applied. Toughness is a measurement of the resistance of the material to propagation of a crack or extension of damage to the point of failure. Even though engineering grade ceramics have superior strength and toughness at relatively low temperatures, they are relatively poor in thermal shock resistance (both strength and toughness) and many grades, such as but not limited to borides, carbides, and nitrides are not chemically stable at high temperature. Many are also not suitable for use at the high temperatures encountered with some pyrolysis reactions.
The second category of ceramic materials is insulation grade ceramics, which are typified by relatively high porosity. Many may have fibrous crystalline grain structures and are more porous than engineering grade ceramics, have lower density, and have lower thermal conductivity than engineering grade ceramics. Insulating monolithic ceramics and composite ceramics are often fabricated into various forms such as rigid boards, cylinders, papers, felts, textiles, blankets, and moldables. Many are primarily used for thermal insulation at elevated temperatures, such as up to 1700° C. A broad range of porosities and pore sizes can be produced, depending on the intended application, but in general, insulation is grade ceramics tend to be relatively porous as compared to engineering grade ceramics. Porous ceramics have many open or closed internal pores that provide the thermal barrier properties. Often, quite porous ceramics, such as those having porosity of greater than 50 vol. % and commonly even in excess of 90 vol. %, are used for thermal insulation where extremely low thermal conductivity (<0.08 W/m·K) is required. However, insulation grade ceramics typically lack the structural strength and functional toughness needed for the internal components of many pyrolysis reactors and processes. Insulation grade ceramics typically are recognized as having a flexural strength or toughness of less than about 4 Kpsi (27.6 MPa) and often of less than even 1 Kpsi (6.9 MPa). Also, the insulation properties of porous ceramics may tend to degrade as the pores may fill with coke accumulation.
The third generally recognized category of ceramic materials is refractory grade ceramics. Many refractory grade ceramics typically have porosity, strength, and toughness properties intermediate to such properties in engineering grade and insulation grade. Refractory grade ceramics typically have thermal shock resistance properties similar to some insulation grade ceramics but higher than engineering grade ceramics. Conversely, refractory grade ceramics typically lack the strength and toughness of engineering grades ceramics, but which properties exceed those of insulation grade ceramics. However, typically as strength increases, thermal shock resistance and related properties are compromised. All relevant properties must be considered when selecting a ceramic for a particular application.
As compared to insulation grade ceramics, refractory grade ceramics tend to be stronger across broader temperature ranges. Refractory grade ceramics also generally tend to be more resistant to thermal shock than engineering grade ceramics. However, while some ceramics tend to be somewhat inert or chemically stable at moderately elevated temperatures, many ceramics become chemically and/or structurally unstable at severely elevated temperatures, tending to degrade and corrode within undesirably short periods of time, rendering them unsuitable for some applications. Exemplary chemically and/or thermally unstable ceramics include certain silicas, aluminas, borides, carbides, and nitrides. Also, some refractory grade ceramics are known to possess lower thermal conductivities and coefficients of expansion than certain other refractory or engineering grade ceramics. Refractory grade ceramics are also known to undergo alterations in crystalline structure at elevated temperatures. Such alterations can result in changes in bulk volume which can result in creation of stress fractures and/or cleavage planes which can reduce the material's strength or performance properties.
Some advanced engineering ceramics, such as aluminas, zirconias, and silica, such as SiC and Si3N4, also provide superior strength, but their thermal shock resistance in grossly inadequate. Moreover, these silicon based ceramics can not be used at high temperatures (i.e. >1500° C.) due to high temperature oxidation issues. On the other end of the spectrum lie the insulation grade ceramics. These ceramics offer excellent thermal shock resistance, but they fall quite short of the required strength performance.
Zirconia is a crystalline material that is commonly used in certain ceramics, also having thermal application. However, zirconia undergoes a crystalline change at different temperatures in the way its atoms are stacked (polymorphic transformation). Zirconia has a monoclinic crystal structure between room temperature and about 1200° C. Above about 1200° C., zirconia converts to a tetragonal crystal structure. At a still higher temperature, such as above 2370° C., zirconia changes from tetragonal to cubic structure and melts at 2715° C. These transformations are accompanied by greater than one percent volumetric shrinkage during heating and equivalent expansion during cooling. The volumetric changes associated with alterations in crystalline structure can produce crystalline fractures or cleavages along grain boundaries. In polycrystalline zirconia, this tetragonal-monoclinic transition results in a reduction in strength and potential catastrophic failure of the component. Stabilizers, such as yttria and some metal oxides are can be into the crystal structure to arrest or prevent the crystalline shifts, rending the crystal structure across a more broad temperature spectrum.
However, it has recently been learned that extended exposure of stabilized ceramic components, such as but not limited to stabilized zirconias, to high temperature processes and severe environments can result in gradual evacuation or loss of the stabilizer component from the ceramic crystals. This loss undesirably results in progressing temperature-related re-alteration of the crystal structure over time, further leading to onset of the aforementioned cleaving and fracturing problems. Such stabilizer material loss and crystal alteration result in a corresponding degradation and reduction in life expectancy of the component, due to compromised performance properties.
The pyrolysis art needs a stabilized ceramic composition or material that provides the desirable set of performance properties and that can sustain those properties for a commercially meaningful period of use, by resisting loss of stabilizer, maintaining crystalline stability, and enduring prolonged exposure to high severity temperatures, substantial temperature swing cycles, cyclic flows of combustion and reaction materials. The desired materials must concurrently provide the needed structural integrity, crystalline stability, is relatively high heat transfer capability, and chemical inertness required for large scale, commercial, high productivity applications. Unavailability of such materials, components, and associated processes has been one of the most critical impediments against large scale, commercial adoption and application of many high temperature pyrolysis and chemistry processes and apparatus.